This is a nasty thriller that would have suited Hitchcock. Joe Penny is the impostor and really bad guy, who declares himself as nothing less from the beginning. The interest of the film is to follow and expect his gradual exposure. By chance he chooses as his next victim a brilliant and successful lawyer (Veronica Hamill) who is beautiful enough, and you expect him to betray himself by seducing her, but he sticks to his role play as her long lost and stipulated brother, so he lets her alone to target others, until he finds the occasion to move in for a definite settlement.
It's a most unpleasant feature getting nastier all the time, while you get caught up with it with the same kind of obsessive interest as with Hitchcock's "Shadow of a Doubt". The problem here is that Veronica Hamill has no doubts at all and refuses to listen to those off others, until she reaches hard evidence. That's a very logical procedure when you are subject to a meticulously calculated deceit - you wish to believe in innocence as long as it is convincing, until it is proved the opposite. Many have experienced this painful process, so the film is permanently actual.